Binary convolutional coding has been applied to the US Digital Cellular (USDC) and Japan Digital Cellular (JDC) systems to provide improved bit error rate (BER) performance. Convolutional codes are typically decoded using the Viterbi algorithm, which can utilize either hard-decision or soft-decision information. A hard-decision decoder operates on receiver bit decisions and uses Hamming distance as its confidence metric. Soft-decision decoding, on the other hand, operates on unquantized demodulator output values. Because it utilizes available channel quality information, soft-decision decoding provides superior BER performance relative to hard-decision decoding.
For a linear receiver, soft-decision information is typically generated by sampling the demodulator output at the symbol rate. This provides both amplitude and phase information which the Viterbi decoder can utilize in forming its confidence metric. In addition to requiring a linear receiver, this approach has another disadvantage: it is sensitive to the removal of some or all of the fading envelope due to automatic gain control (AGC). If the AGC gain varies over the effective memory length of the decoder, the detector output has to be remodulated with the removed fading envelope in order to obtain the full soft-decision performance gain.
For digital phase modulation schemes such as .pi./4 QPSK, a limited receiver structure is often preferred. In this case, the sampled demodulator output alone would provide inferior soft-decision decoding information because all amplitude information (including the fading envelope) has been removed. As a result, the demodulator output is typically scaled by measured received signal strength information (RSSI). This approach, however, requires additional circuitry (including an analog-to-digital converter to digitize the RSSI) and complicates the interface between the demodulator and the rest of the receiver.
Accordingly, there is a need for a method and apparatus for generating effective soft-decision information without signal amplitude information.